Hydrodynamic type porous oil-impregnated bearing

ABSTRACT

The porous oil-impregnated bearing  1  comprises a bearing body  1   a  made of a porous material, and oil retained in the pores of the bearing body  1   a  by impregnation with lubricating oil or lubricating grease. The inner peripheral surface of the bearing body  1   a  is formed with a bearing surface  1   b  opposed to an outer peripheral surface of a shaft to be supported, with a bearing clearance defined therebetween. The bearing surface  1   b  has a first region m 1  in which a plurality of hydrodynamic pressure generating grooves  1   c  inclined in one direction with respect to the axial direction are circumferentially disposed, a second region m 2  which is axially spaced from said first region m 1  and in which a plurality of hydrodynamic pressure generating grooves  1   c  inclined in the other direction with respect to the axial direction are circumferentially disposed, and an annular smooth region n disposed between the first and second regions m 1  and m 2.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a hydrodynamic type porousoil-impregnated bearing being impregnated with lubricating oil orlubricating grease in a bearing body of porous substance, such assintered metal, to have a self-lubricating function, supporting a slidesurface of a shaft in a non-contact manner by a lubricating oil filmproduced in a bearing clearance due to hydrodynamic function ofhydrodynamic pressure generating grooves in a bearing surface. Thebearing of the invention is suitable for use particularly in machinesand instruments of which high rotation accuracy at high speed isrequired, such as spindle motors for polygon mirror of laser beamprinter (LBP), magnetic disk drives (HDDs), or the like, and in machinesand instruments which are driven at high speed with a large imbalanceload produced in that a disk is mounted thereon, such as spindle motorsfor DVD-ROM, or the like.

[0002] In such small-sized spindle motors associated withinformation-handling devices, improved rotation performance and costreduction are required, as a means therefor, possibility of changingbearings for the spindle from a rolling bearing to a porousoil-impregnated bearing has been investigated. However, since a porousoil-impregnated bearing is a kind of cylindrical bearing, it tends toproduce unstable vibrations where the shaft eccentricity is small,inducing the so-called whirl in which the shaft is subjected to arevolving vibration at a rate which is half the rotary speed.Accordingly, it has heretofore been attempted to form hydrodynamicpressure generating grooves, such as the herringbone or spiral shape, ina bearing surface, so as to produce a lubricating oil film in a bearingclearance by the function of the hydrodynamic pressure generatinggrooves which accompanies the rotation of the shaft, to thereby supportthe shaft in a noncontact manner (hydrodynamic type porousoil-impregnated bearing).

[0003] A porous oil-impregnated bearing being formed hydrodynamicpressure generating grooves in a bearing surface is disclosed inJapanese Utility Model Koukoku Shouwa 63-19627. In this prior art, aregion of the hydrodynamic pressure generating grooves in the bearingsurface is worked to seal surface openings thereon. Such construction,however, has the following drawback. Since the surface openings on theregion of the hydrodynamic pressure generating grooves completelysealed, the circulation of oil, which is the greatest feature of theporous oil-impregnated bearing, is obstructed. Therefore, the oil whichhas been exuded in the bearing clearance is forced into the bentportions of the groove region by the action of the hydrodynamic pressuregenerating grooves and stays there. A great shearing action is presentin the bearing clearance, and this shearing force and frictional heatcause the oil staying in the groove region to be denatured, while a risein temperature tends to accelerate oxidative deterioration of the oil.Therefore, the bearing life is shortened. On the other hand, besidesplastic processing, it has been proposed to employ coating or the likeas another means for applying a surface treatment, however, it isnecessary that the thickness of such coating film be less than thegroove depth, and it is very difficult to apply a coating film which issome μ m thick solely to the groove region.

[0004] In order to secure the rotation accuracy of the shaft, aplurality of bearings, e.g., two bearings, are usually used. Further,bearings are used mostly by being pressed into a housing. Thus, tosecure a substantial alignment of the two bearings, there has beenemployed a method in which two bearings are simultaneously pressed intothe housing after a correcting pin is inserted into the housing. In thecase of a bearing having hydrodynamic pressure generating grooves formedin the bearing surface, if forcible correction is made by using thecorrection pin, this will result in the correction pin cutting into thehydrodynamic pressure generating grooves in the bearing surface tocollapse said grooves, making it impossible to obtain a stabilizedhydrodynamic effect. On the other hand, the operation of press-fittingwithout using the correction pin will fail to provide the necessaryalignment between the bearings. Further, Japanese Patent Kokai Heisei2-107705 discloses an arrangement in which two bearing surfaces areformed in axially spaced from each other and in which a region betweenthe bearing surfaces has a greater diameter than that of the bearingsurfaces. This arrangement, though free from the aforesaid problems inpractice, cannot prevent the unstable vibrations, such as whirl, becauseof the lack of hydrodynamic pressure generating grooves in the bearingsurfaces.

[0005] As for a method of forming hydrodynamic pressure generatinggrooves in bearing surfaces, such a method has been a known thatcomprises the steps of inserting into an inner peripheral surface of abearing blank a shaft-like jig which holds a plurality ofcircumferencially equispaced balls harder than the bearing blank,imparting a spiral movement to the balls through the rotation andadvance of the jig while pressing the balls work a region ofhydrodynamic pressure generating grooves method of , which method(Japanese Patent 2541208). In this method, the blank bulges in a regionadjacent the hydrodynamic pressure generating grooves during forming,and such bulge has to be removed as by lathing or reaming (JapanesePatent Kokai Heisei 8-232958). For this reason, the number ofmanufacturing steps increases. Further, a driving mechanism and anadvancing mechanism for the jig are required, thus complicating themanufacturing equipment.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to secure the appropriatecirculation of oil between the interior of the bearing body and thebearing clearance to suppress the deterioration of the oil in thebearing clearance, thereby increasing the bearing life, and improvingthe effect of formation of lubricating oil film in the bearingclearance, thus increasing the bearing rigidity and minimizing the shaftdeflection due to imbalance load or the like.

[0007] Another object of the invention is to provide an arrangementwhich is capable of preventing unstable vibrations such as whirl andeliminating the inconveniences (such as the loss of shape ofhydrodynamic pressure generating grooves, and axial misalignment) whichare involved in the installing operation.

[0008] A further object of the invention is to provide a productionmethod which facilitates the forming of a bearing surface havinginclined hydrodynamic pressure generating grooves by using simpleequipment and fewer steps and with high accuracy.

[0009] To achieve said objects, the invention provides a hydrodynamictype porous oil-impregnated bearing comprising a porous bearing bodybeing formed with bearing surface on an inner peripheral surfacethereof, and oil retained in pores of the bearing body by impregnationof lubricating oil or lubricating grease, wherein the bearing surfacehas a first region in which a plurality of hydrodynamic pressuregenerating grooves inclined in one direction with respect to the axialdirection are circumferentially disposed, a second region which isaxially spaced from the first region and in which a plurality ofhydrodynamic pressure generating grooves inclined in the other directionwith respect to the axial direction are circumferentially disposed, andan annular smooth region positioned between the first and secondregions. The bearing surface of the bearing body is opposed to an outerperipheral surface of a shaft to be supported, with a bearing clearancedefined therebetween. When a relative rotation occurs between thebearing body and the shaft, the hydrodynamic pressure generating groovesmutually reversely disposed in the first and second regions of thebearing surface cause the oil in the bearing clearance to be drawn tothe annular smooth region and collect in the latter, so that the oilfilm pressure in the smooth region is increased. For this reason, theeffect of formation of lubricating oil film is high. Further, since thesmooth region has no groove formed therein, the bearing rigidity is highas compared with the construction in which hydrodynamic pressuregenerating grooves axially continuous. Therefore, the shaft deflectioncan be minimized. Further, it is possible to avoid the lubricating oilfilm distribution becoming nonuniform owing to variations in surfaceopenings on the bearing surface. By the term “surface openings” is meantthose portions of pores of a porous body which open to an outer surfacethereof. In the present invention, the surface openings are present inthe entire region of the bearing surface including the region formedwith the hydrodynamic pressure generating grooves.

[0010] Percentage of area of surface openings in the smooth region ofthe bearing surface is preferably smaller than that of the first andsecond regions. By the term “percentage of area of surface openings” ismeant the proportion of the total area of the surface openings in unitarea of the outer surface. As a result, since the oil which is broughttogether in the smooth region by the hydrodynamic pressure generatinggrooves can hardly escape into the interior of the bearing body throughthe surface openings on the smooth region, the capacity of the producedlubricating oil film can be increased. Further, since an outerperipheral surface of the shaft is supported in a non-contact mannermainly by the lubricating oil film formed of the oil collected in theannular smooth region, the bearing rigidity is high.

[0011] The percentage of area of sureface openings is in the range of5-30%, desirably 5-20%, for the first and second regions and 2-20%,desirably 2-15%, for the smooth region. If the percentage of area ofsurface openings on the first and second regions is less than 5%, theamount of oil to be fed from the interior of the bearing body to thebearing clearance decreases, resulting in insufficient formation oflubricating oil film. Reversely, if it exceeds 30%, the amount of oilwhich escapes into the interior of the bearing body becomes excessive,resulting in insufficient formation of lubricating oil films on thesmooth region. Further, if the percentage of area of surface openings onthe smooth region is less than 2%, the production of the bearing becomesdifficult, leading to an increase in costs. Reversely, if it exceeds20%, the amount of oil which escapes into the interior of the bearingbody becomes excessive, resulting in insufficient formation oflubricating oil film.

[0012] In order to enhance the effect of formation of lubricating oilfilm on the smooth region, it is preferable that the hydrodynamicpressure generating grooves in the first region and those in thesecond-region be symmetric with respect to the axial central region ofthe bearing surface.

[0013] At the start or stoppage of rotation, the outer peripheralsurface of the shaft comes into instantaneously contact with the bearingsurface of the bearing. At this time, they come into contact with eachother in the axial end region of the bearing surface. Therefore, bytapering the axial opposite sides of the bearing surface such that theinner diameter increases toward the bearing ends (see FIG. 7), the areaof their contact is increased when the apparatus is started or stopped,so that the non-contact state can be instantaneously established. Thefirst and second regions may be tapered throughout or portions(associated with the bearing ends) of each of the first and secondregions may be tapered. In addition, the area of the bearing surfaceother than the tapered surface is parallel with the axis.

[0014] In this case, the ratio of an increment Δc in the inner diameterfrom the smooth region to the end of the bearing to the shaft diameter Dis Δc/D=1/3000-1/200, more desirably, Δc/D=1/3000-1/500. If Δc/D is lessthan 1/3000, the resulting taper is too small to prevent instantaneouscontact, and if Δc/D is greater than 1/200, the resulting taper is toolarge to provide a useful hydrodynamic effect.

[0015] It is possible to provide an arrangement comprising a porousbearing body being formed with a plurality of axially spaced bearingsurfaces on an inner peripheral surface thereof, at least one of theplurality of bearing surfaces having inclined hydrodynamic pressuregenerating grooves, the inner diameter of the region between the bearingsurfaces being greater than that of the bearing surfaces, and oilretained in the pores of the bearing body by impregnation of lubricatingoil or lubricating grease. Such formation of a plurality of bearingsurfaces in a single bearing solves the problem of axial alignmentinherent in the case where a plurality of bearings are incorporated asin the prior art. More particularly, since the plurality of bearingsurfaces are formed in a single bearing, there is no need to use acorrecting pin to obtain axial alignment as in the case of prior art,and the loss of shape of the hydrodynamic pressure generating groovesdue the use of such correcting pin does not occur, of course. Theformation of inclined hydrodynamic pressure generating grooves in atleast one bearing surface effectively prevents unstable vibrations suchas whirl.

[0016] Provision of a level difference in the boundary between thebearing surface and the region between the bearing surfaces makes itpossible to effectively reduce the torque loss in the region between thebearing surfaces.

[0017] If the axial section of the region between the bearing surfacesis drawn with a curve which continuous to the bearing surfaces, oilwhich exudes from the surface openings on the region between the bearingsurfaces flows axially along such region, making it easier to feed theoil to the bearing surface, a fact which means effective use of oil andenhancement of formation of lubricating oil film.

[0018] The axial section of the region between the bearing surfaces maybe drawn with an arc which is greatest in the middle of the region. Theoil which has exuded from the surface openings on the region can beeasily fed to the bearing surfaces on the opposite sides.

[0019] The outer diameter of an outer portion of the bearing bodycorresponding to at least one bearing surface is determined to besmaller than the outer diameter of an outer portion of the bearing bodycorresponding to the region between the bearing surfaces, whereby whenthe bearing body is press-fitted in a housing, deformation of thebearing surfaces under the press-fitting pressure can be prevented orreduced.

[0020] The bearing surface having inclined hydrodynamic pressuregenerating grooves can be formed by the following method: the methodcomprises the steps of inserting a forming pattern in an innerperipheral surface of a cylindrical porous blank, the forming patternhaving a first forming portion for forming a region of hydrodynamicpressure generating grooves and a second forming portion for forming theother regions in the bearing, applying a compacting pressure to theporous blank to press the inner peripheral surface of the porous blankagainst the forming pattern, thereby simultaneously forming the regionof hydrodynamic pressure generating grooves and the other region in thebearing surface on the inner peripheral surface of the porous blank.Alternatively, disposing the forming pattern in a die, filling powdermetal material between the forming pattern and the die, applying acompacting pressure to the powder metal material to form a cylindricalcompacted body, while simultaneously forming the region of hydrodynamicpressure generating grooves and the other region in the bearing surfaceon the inner peripheral surface of the compacted body. Release of theforming pattern can be effected by utilizing the spring-back of theporous blank due to removal of the compacting presuure, or by utilizingthe spring-back of the compacted body due to removal of the compactingpresuure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a longitudinal sectional view showing an embodiment of ahydrodynamic type porous oil-impregnated bearing;

[0022]FIG. 2 is a longitudinal sectional view conceptually showing amotor having the hydrodynamic type porous oil-impregnated bearing of theembodiment;

[0023]FIG. 3 is a view schematically showing the flow of oil in theaxial section when a shaft is supported in a non-contact manner by thehydrodynamic type porous oil-impregnated bearing;

[0024]FIG. 4 is a longitudinal sectional view showing another embodimentcomparative of a hydrodynamic type porous oil-impregnated bearing;

[0025]FIG. 5 is a graph showing the results of comparative tests onshaft deflection when the embodied articles and the comparative articleare used (in the case where the amount of imbalance is small);

[0026]FIG. 6 is a graph showing the results of comparative tests onshaft deflection when the embodied articles and the comparative articleare used (in the case where the amount of imbalance is large);

[0027]FIG. 7 is a longitudinal sectional view showing another embodimentof a hydrodynamic type porous oil-impregnated bearing;

[0028]FIG. 8 is a graph showing the results of comparative tests on theoil film forming state at the start of rotation when the embodiedarticle and the comparative article are used;

[0029]FIG. 9 is a fragmentary enlarged cross sectional view of thehydrodynamic type porous oil-impregnated bearing;

[0030]FIG. 10 is a longitudinal sectional view schematically showing howthe oil is spattered when a shaft is supported in a non-contact mannerby the hydrodynamic type porous oil-impregnated bearing;

[0031]FIG. 11 is a longitudinal sectional view showing a sintered metalblank to be used in an embodiment of the production method;

[0032]FIG. 12A is a longitudinal sectional view showing the outline of aforming device used for forming a bearing surface, and FIG. 12B is aside view showing a die for forming a bearing surface;

[0033] FIGS. 13-15 are views showing the forming steps for a bearingsurface;

[0034]FIG. 16 is a graph showing the relation between the innerclearance and outer interference, and the amount of spring- back;

[0035]FIG. 17 is a graph showing the results of comparative tests onshaft deflection when a cyrindrical bearing and a hydrodynamic typeporous oil-impregnated bearing produced by the production method of theembodiment are used;

[0036]FIG. 18 is a longitudinal sectional view conceptually showing atesting device used for the comparative tests shown in FIG. 17;

[0037]FIG. 19 is a longitudinal sectional view showing an embodiment ofa hydrodynamic type porous oil-impregnated bearing having a plurality ofbearing surfaces;

[0038]FIG. 20 is a view schematically showing the flow of oil in theaxial section when a shaft is supported in a non-contact manner by thehydrodynamic type porous oil-impregnated bearing shown in FIG. 19;

[0039]FIG. 21 is a graph showing the relation between the percentage ofarea of surface openings on the bearing surface and the kinematicviscosity of oil;

[0040]FIGS. 22 and 24 are graphs showing the results of evaluation testson shaft deflection; and

[0041]FIG. 23 is a longitudinal sectional view showing anotherembodiment of a hydrodynamic type porous oil-impregnated bearing havinga plurality of bearing surfaces.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] Embodiments of the present invention will now be described.

[0043]FIG. 1 shows by way of example an embodiment of a hydrodynamictype porous oil-impregnated bearing. This hydrodynamic type porousoil-impregnated bearing 1 is used, for example, in connection with ascanner motor for a laser beam printer shown in FIG. 2, to support aspindle shaft 2 for rotation with respect to a housing 4, in anon-contact manner, the spindle shaft 2 being rotated at high speed bymagnetic excitation force between a rotor 3 and a stator.

[0044] The porous oil-impregnated bearing 1 comprises a bearing body 1 amade of a porous material, e.g., a sintered metal containing copper oriron, or both as a main component, and oil retained in the pores of thebearing body 1 a by impregnation with lubricating oil or lubricatinggrease. The bearing body preferably contains copper in 20-95 wt %, andhas density of 6.4-7.2 g/cm³.

[0045] The inner peripheral surface of the bearing body 1 a is formedwith a bearing surface 1 b opposed to an outer peripheral surface of ashaft to be supported, with a bearing clearance defined therebetween,the bearing surface 1 b being formed with inclined hydrodynamic pressuregenerating grooves 1 c. The bearing surface 1 b in this embodimentcomprises a first region m1 in which a plurality of hydrodynamicpressure generating grooves 1 c inclined in one direction with respectto the axial direction are circumferentially disposed, a second regionm2 which is axially spaced from said first region m1 and in which aplurality of hydrodynamic pressure generating grooves 1 c inclined inthe other direction with respect to the axial direction arecircumferentially disposed, and an annular smooth region n disposedbetween the first and second regions m1 and m2. The ribs 1 d (theregions between the hydrodynamic pressure generating grooves 1 c) of thefirst region m1 and the ribs 1 d (the regions between the hydrodynamicpressure generating grooves 1 c) of the second region m2 are continuousto the smooth region n. The hydrodynamic pressure generating grooves 1 cof the first region m1 and the hydrodynamic pressure generating grooves1 c of the second region m2 are symmetrical with respect to the axialcenterline L of the bearing surface 1 b. The bearing surface 1 b hassurface openings distributed over the entire area including the regionwhere the hydrodynamic pressure generating grooves 1 c are formed, itbeing arranged that the oil is circulated between the interior of thebearing body 1 a and the bearing clearance through the surface openingsof the bearing body 1 a including the bearing surface 1 b so as tosupport the outer peripheral surface of the shaft in a non-contactmanner with respect to the bearing surface 1 b. It is advisable that thepercentage of area of surface openings on the smooth region n be equalor lower than that of the first and second regions m1 and m2.

[0046] When relative rotation takes place between the bearing body 1 aand the shaft, the mutually reversely directed, inclined hydrodynamicpressure generating grooves 1 c formed in the first and second regionsm1 and m2 draw the oil in the bearing clearance toward the smooth regionn, whereby the oil is collected on the smooth region n; therefore, theoil film pressure on the smooth region n is increased. Thus, the effectof formation of lubricating oil film is high. Furthermore, not only theribs 1 d but also the smooth region n provides a support surface tosupport the shaft; thus, the area of support is increased and thebearing rigidity is high. The ratio r of the axial width of the smoothregion n to the bearing width when the latter is taken to be 1 ispreferably in the range of r=0.1-0.6, more desirably, r=0.2-0.4. If r isless than 0.1 with respect to the bearing width of 1, the effect to beobtained by reason of the provision of the smooth region n (improvedhydrodynamic action, and increased bearing rigidity) fails to manifestitself, whereas if it is greater than 0.6 with respect to the bearingwidth of 1, the regions where the hydrodynamic pressure generatinggrooves 1 c are formed are decreased, exhibiting less force which urgesthe oil to the axial central region, thus failing to develop thehydrodynamic effect. In addition, the hydrodynamic pressure generatinggrooves 1 c are shown by way of example in a herringbone form; however,they may be in any form so long as they are inclined with respect to theaxis. For example, they may be in a spiral form.

[0047]FIG. 3 shows the flow of oil 0 in the axial section when the shaft2 is supported by the porous oil-impregnated bearing 1 of the abovementioned construction. With the rotation of the shaft 2, the oil 0retained in the pores of the bearing body 1 a exudes from the axialopposite sides of the bearing surface 1 b (and the chamfers) into thebearing clearance, and is drawn toward the axial center of the bearingclearance by the hydrodynamic pressure generating grooves. The pressureof luburicating oil film present in the bearing clearance is increasedby such action of drawing the oil 0 (the hydrodynamic action). Theluburicating oil film formed in the bearing clearance supports the shaft2 in a non-contact manner with respect to the bearing surface 1 bwithout producing unstable vibrations such as whirl. The oil 0 exudingto flow into the bearing clearance flows back into the bearing body 1 athrough the surface openings on the bearing surface 1 b under thepressure produced with the rotation of the shaft 2, then circulating inthe interior of the bearing body 1 a, again exuding to flow into thebearing clearance through the surface openings on the bearing surface 1b (and the chamfers).

[0048] Generally, since it is difficult to make uniform the distributionof the surface openings on the bearing surface, large and small surfaceopenings are present on the bearing surface. Therefore, the amount ofoil which returns to the interior of the bearing body differs from placeto place. As a result, in the place where oil escapes with ease, oilfilms hardly form, whereas in the place where oil hardly escapes, oilfilms form with ease, resulting in the oil film in the bearing clearancehaving a nonuniform distribution, making it sometimes impossible toobtain a stabilized hydrodynamic effect. In this connection, the porousoil-impregnated bearing 1 of this embodiment has the annular smoothregion n between the first and second regions m1 and m2, and in thesmooth region n, the distribution of the surface openings is easier touniformly control. Further, in the first and second regions m1 and m2the flow of oil in the direction of the grooves is dominant, while inthe smooth region n there is a circumferential flow of oil, so that evenif there are large surface openings, oil is successively supplied andhence the rate at which the formation of oil films is reduced is muchlower.

[0049] The hydrodynamic type porous oil-impregnated bearing 11 shown inFIG. 4 has a bearing surface 11 b being different from the bearingsurface 1 b of the above mentioned embodiment in shape. The bearingsurface 11 b has a first region in which a plurality of hydrodynamicpressure generating grooves 11 c inclined in one direction with respectto the axial direction are circumferentially disposed, a second regionwhich is axially continuous to the first region and in which a pluralityof hydrodynamic pressure generating grooves 11 c inclined in the otherdirection with respect to the axial direction are circumferentiallydisposed. The surface openings are distributed on the entire region ofthe bearing surface 11 b including regions of the hydrodynamic pressuregenerating grooves 11 c. Under a condition in which there is a littleimbalance of a rotary body so that the bearing rigidity is not requiredas a impotant characteristic of a bearing, a bearing surface which hasaxially continuous hydrodynamic pressure generating grooves, such as theabove bearing surface 11 b, is preferable according to circumstances.

[0050] Various test bearings were incorporated into a small-sizedspindle motor as shown in FIG. 2 and tested for shaft deflection. Theresults are shown in FIGS. 5 and 6. FIG. 5 shows the results obtainedwhen almost no imbalance load is applied (imbalance load: 50 mg·cm orless), and FIG. 6 shows the results obtained when large imbalance loadis applied (imbalance load: 1 g·cm). As for the test bearings, use wasmade of embodied articles A(▪) and B(◯) of the arrangement shown in FIG.1, C(▴) of the arrangement shown in FIG. 4, and a cylindrical bearing (aporous oil-impregnated bearing having no hydrodynamic pressuregenerating grooves formed in the bearing surface: ). The specificationsof the test bearings are as follows. The size of the cylindrical bearing(), the size of the bearing clearance and other specifications than theshape of the bearing surface are the same as the embodied articles.

[0051] [Embodied Article A:▪]

[0052] Size: inner dia. ø3×outer dia. ø6×width 3 mm

[0053] Bearing clearance: 4 μm

[0054] Percentage of area of surface openings on bearing surface: 20%

[0055] *Specifications of hydrodynamic pressure generating grooves

[0056] Groove depth: 3 μm

[0057] Number of grooves: 8 for first region, 8 for second region

[0058] Angle of inclination of grooves: 20 degrees

[0059] Ratio of width of grooves to width of ribs: 1

[0060] Width of bearing surface: 2.4 mm (with 0.3 mm chamfers on bothsides)

[0061] Width of first and second regions: 0.9 mm

[0062] Width of smooth region: 0.6 mm

[0063] [Embodied article B: ◯]

[0064] Size: inner dia. φ3×outer dia. φ6×width 3 mm

[0065] Bearing clearance: 4 μm

[0066] Percentage of area of surface openings on bearing surface: 20%for first and second regions, 10% for smooth region

[0067] *Specifications of hydrodynamic pressure generating grooves

[0068] Groove depth: 3 μm

[0069] Number of grooves: 8 for first region, 8 for second region

[0070] Angle of inclination of grooves: 20 degrees

[0071] Ratio of width of grooves to width of ribs: 1

[0072] Width of bearing surface: 2.4 mm (with 0.3 mm chamfers on bothsides)

[0073] Width of first and second regions: 0.9 mm

[0074] Width of smooth region: 0.6 mm

[0075] [Embodied article C: ▴]

[0076] Size: inner dia. φ3×outer dia. φ6×width 3 mm

[0077] Bearing clearance: 4 μm

[0078] Percentage of area of surface openings in bearing surface: 20%

[0079] *Specifications of hydrodynamic pressure generating grooves

[0080] Groove depth: 3 μm

[0081] Number of grooves: 8

[0082] Angle of inclination of grooves: 20 degrees

[0083] Ratio of width of grooves to width of ribs: 1

[0084] Width of bearing surface: 2.4 mm (with 0.3 mm chamfers on bothsides)

[0085] The embodied article C(▴) produced less shaft deflection than thecylindrical bearing () but more shaft deflection than the embodiedarticles A, B(▪, ◯), and particularly in the region of higher imbalanceload and higher rpm, it produced a large increment in shaft deflection.The embodied articles A, B(▪, ◯) produced less shaft deflectionirrespective of the size of the imbalance load, and particularly in theregion of higher rpm, they produced only a slight increment in shaftdeflection. Therefore, the embodied articles A, B(▪, ◯) can minimizeshaft deflection not only for those devices which are subjected to lowimbalance load, such as LBP motors but also for those devices which aresubjected to high imbalance load when a disk is mounted thereon, such asDVD-ROM motors.

[0086] Next, as shown in FIG. 7, a bearing (an embodied article {circleover (2)}) in which the axial opposite sides of the bearing surface 1 bare tapered such that the inner diameter was increased toward thebearing ends and the cylindrical bearing ({circle over (1)}) are testedto find the frequency of contact with the shaft at the start of rotationon the basis of the oil film formation percentage. The results are shownin FIG. 8. In addition, the rpm of the shaft was 6,000.

[0087] In the case of the cyrindrical bearing ({circle over (1)}), sinceits oil film formation percentage at the start of rotation was low, itsfrequency of contact with the shaft was high. The reason is thatimmediately after the start of rotation, the oil in the bearingclearance is not affluent and the shaft precesses (swings), so that atthe sides of the bearing surface, the shaft and the bearing edgewiseabut against each other, thus occasioning contact. In contrast, theembodied article ({circle over (2)}) had undergone no contact with theshaft since the rotation started and instead an oil film was instantlyformed therein. The reason is that since the axial opposite sides of thebearing surface 1 b are tapered, the edgewise abutment between the shaftand bearing is avoided.

[0088] In addition, there is an optimum range in the ratio of thehydrodynamic pressure generating groove depth to the radial clearance,outside which range the hydrodynamic effect is greatly reduced. If c/his in the range of 0.5-5.0 (see FIG., 9), a high rotation accuracy whichcauses no problems in practice can be maintained.

[0089] Further, although porous oil-impregnated bearings are usuallyused without being fed with oil, gradual exhaustion or outflow of theinternally retained oil due to spattering and evaporation of the oilcannot be avoided. When the oil has been exhausted, the range of oilfilm formation decreases, leading to degradation of the rotationaccuracy, such as shaft deflection. Particularly, a shaft is used oftenin its vertical position, as shown in FIG. 10, and in the case of alaser beam printer motor which is used at a high speed of 10,000 rpm,the oil retained internally of the bearing tends to flow out under theaction of centrifugal force, so that it has been difficult to maintainthe performance, such as the formation of oil films. In the case of LBand HDD, discontinuation of oil films is fatal to the maintenance ofhigh rotation accuracy. In the case of a single porous oil-impregnatedbearing, particularly when the shaft is rotated at high speed, the oil,taking in the ambient air, is circulated in the bearing, sometimesresulting in the air migrating into the bearing clearance. To preventthe migration of air, it is effective to place an oil re-feeding memberin close contact with the bearing body, so as to re-feed oil from theoil re-feeding member as soon as even very few empty pores are created.Placement of an oil re-feeding member brings about not only the effectof prolonging life but also the effect of maintaining an oil film whichis necessary for maintaining high accuracy. The oil re-feeding memberused in close contact with the bearing body may be in the known form ofa porous body, such as metal or resin, or a fibrous material, such asfelt, impregnated with oil, but it is preferable to use a solidlubricating composition which has the nature of gradually continuouslyexuding the internally retained oil to the surface at temperatures of atleast 20C. It is recommendable to use, e.g., a solid resin lubricatingcomposition prepared by melting a mixture of lubricating oil orlubricating grease and superhigh molecular weight polyethylene powder,and cooling the melt to solidify the latter. This solid resinlubricating composition continuously exudes the retained oil at not lessthan ordinary temperatures, making it possible to continuously re-feedoil to the bearing. Further, this solid resin lubricating compositioncan be mass-produced at low cost and is easy to handle.

[0090] Thus, if a solid resin lubricating composition which graduallycontinuously exudes oil to the surface even when left to stand at notless than ordinary temperatures is placed in close contact with thesurface of the bearing, then even if the oil in the bearing flows away,oil is re-fed into the interior of the bearing by the capillary actionwhich occurs in the pores of the bearing body, so that a satisfactoryhydrodynamic oil film can be formed at all times. This solid resinlubricating composition can be produced by the following method.

[0091] For example, it is obtained by uniformly mixing a predeterminedamount of lubricating grease or lubricating oil with a predeterminedamount of superhigh molecular weight polyolefin powder, pouring themixture into a die of predetermined shape, and melting the mixture attemperatures not less than the gelling temperature of the superhighmolecular weight polyolefin powder and not more than the dropping pointof lubricating grease if such grease is used, and cooling the mixture atordinary temperatures. The superhigh molecular weight polyolefin powdermay be a powder of polyethylene, polypropylene, or polybutene or acopolymer thereof, or a mixture of these powders, the molecular weightof each powder being so selected that the average molecular weightmeasured by the viscosity method is 1×10⁶-5×10⁶. Polyolefins which arewithin the range of such average molecular weight are superior to lowmolecular weight polyolefins in rigidity and oil retention and willhardly flow even heated to high temperatures. The proportion of suchsuperhigh molecular weight polyolefin in the lubricating composition is95-1 wt %, and the amount depends on the desired degree of bleeding,toughness and hardness of the composition. Therefore, the greater theamount of superhigh molecular weight polyolefin, the higher the hardnessof the gel after dispersion at a predetermined temperature.

[0092] Further, the lubricating grease used in this invention is notparticularly restricted, and may be a soap-thickened ornon-soap-thickened lubricating grease, examples of such lubricatinggrease being lithium soap-diester type, lithium soap-mineral oil type,sodium soap-mineral oil type, aluminum soap-mineral oil type, lithiumsoap-diester mineral oil type, non-soap-diester type, non-soap-mineraloil type, non-soap- polyolester type, and lithium soap-polyolester type.The lubricating oil is not particularly restricted, either, examplesthereof being diester type, mineral oil type, diester mineral oil type,polyolester type, and polyαolefin type. In addition, the base oil forthe lubricating grease or the lubricating oil is desirably the samelubricating oil as that with which the porous oil-impregnated bearing isinitially impregnated, but it may be more or less different therefrom solong as the lubricating characteristics are not impaired.

[0093] Although the melting points of the superhigh molecular weightpolyolefins mentioned above are not constant as they vary according totheir respective average molecular weights, one, e.g., having an averagemolecular weight of 2×10⁶ as measured by the viscosity method has amelting point of 136° C. As for a commercially available one having thesame average molecular weight, there is Mipelon (registered trade mark)XM-220, produced by Mitsui Petrochemical Industries, Ltd., and the like.

[0094] Therefore, when it is desired to disperse superhigh molecularweight polyolefin in the aforesaid lubricating grease or lubricating oiland retain it therein, said materials, after being mixed, are heated toa temperature not less than the gelling temperature of the superhighmolecular weight polyolefin and if lubricating grease is used, to atemperature less than the dropping point thereof, e.g., to 150-200° C.

[0095] Such bearing device can be widely utilized, for example, invarious motors, including laser beam printer polygon mirror motors,magnetic disk drive spindle motors, and DVD-ROM motors, and motors foraxial fans, ventilating fans, electric fans and other electricappliances, electric parts for cars, etc, and their durability can begreatly improved by hydrodynamically supporting the shaft.

[0096] The bearing body 1 a of the porous oil-impregnated bearing 1shown in FIG. 1 can be produced by compacting a metal powder materialcontains copper or iron, or both as a main component, sintering it toobtain a cylindrical sintered metal blank 13 shown in FIG. 11, andsubjecting said blank to sizing→rotation sizing→bearing surface forming.

[0097] The sizing process is a process for sizing the outer and innerperipheral surfaces of the sintered metal blank 13, which is performedby press-fitting the outer peripheral surface of the sintered metalblank 13 in a cylindrical die while press-fitting a sizing pin in theinner peripheral surface. The rotation sizing process is a process inwhich a polygonal sizing pin is press-fitted in the inner peripheralsurface of the sintered metal blank 13 and then the inner peripheralsurface is sized while the sizing pin is rotated. The bearing surfaceforming process is a process in which a forming pattern having a shapecorresponding to the bearing surface 1 b of a finished product 1 a ispressed against the inner peripheral surface of the sintered metal blank13 having said sizing treatment applied thereto to therebysimultaneously form a region of hydrodynamic pressure generating grooves1 c and the other regions (ribs 1 d and annular smooth region n) in thebearing surface 1 b . This process is, for example, as follows.

[0098]FIG. 12A shows by way of example the outline construction of aforming machine used in the bearing surface forming process. This devicecomprises a cylindrical die 20 in which the outer peripheral surface ofthe sintered metal blank 13 is to be press-fitted, a core rod 21 forforming the inner peripheral surface of the sintered metal blank 13, andupper and lower punches 22 and 23 for holding the upper and lower endsurfaces of the sintered metal blank 13. As shown in FIG. 12B, the outerperipheral surface of the core rod 21 is formed with forming pattern 21a in concave-convex form corresponding to the shape of the bearingsurface 1 b of a finished product. The convex portion 21 a 1 of theforming pattern 21 a is to form the region of the hydrodynamic pressuregenerating grooves 1 c in the bearing surface 1 b, while the concaveportion 21 a 2 is to form the other region (ribs 1 d and annular smoothregion n) than the region of the hydrodynamic pressure generatinggrooves 1 c in the bearing surface 1 b. The level difference (depth H,for example 2-5 μm) between the convex and concave portions 21 a 1 and21 a 2 of the forming pattern 21 a is as deep as the hydrodynamicpressure generating grooves 1 c in the bearing surface 1 b, but it isshown considerably exaggerated in the figure.

[0099] Before the sintered metal blank 13 is press-fitted in the die 20,there is an inner clearance T between the inner peripheral surface ofthe sintered metal blank 13 and the forming pattern 21 a of the core rod21 (based on the convex portion 21 a 1). The size (diametrical value) ofthe inner clearance T is, e.g., 50 μm. The press-fit allowance (outerinterference U: diametrical value) for the outer peripheral surface ofthe sintered metal blank 13 with respect to the die 20 is, e.g., 150 μm.

[0100] After the sintered metal blank 13 is placed on the die 20 foralignment, as shown in FIG. 13, the upper punch 22 and core rod 21 arelowered to press-fit the sintered metal blank 13 in the die 20 to urgeit against the lower punch 23, thereby pressing it from above and below.

[0101] The sintered metal blank 13 receives a compacting pressure fromthe die 20 and upper and lower punches 22, 23 and is thereby deformed,with the inner peripheral surface thereof pressed against the formingpattern 21 a of the core rod 21. The amount of compression of the innerperipheral surface of the sintered metal blank 13 is approximately equalto the difference between the outer interference U and the innerclearance T, and the surface layer portion of the sintered metal blank13 extending from the inner peripheral surface to a predetermined depthis pressed by the forming pattern 21 a of the core rod 21, producing aplastic flow which cuts into the forming pattern 21 a. Thereby, theshape of the forming pattern 21 a is transferred to the inner peripheralsurface of the sintered metal blank 13, whereby the bearing surface 1 bis formed to have the shape shown in FIG. 1.

[0102] After the forming of the bearing surface 1 b is completed, asshown in FIG. 14, with the core rod 21 inserted in the sintered metalblank 13, the lower punch 23 and core rod 21 are operatively lifted (thestate of FIG. 14 {circle over (2)}) and the sintered metal blank 13 isextracted from the die 20 (the state of FIG. 14 {circle over (3)}). Whenthe sintered metal blank 13 is extracted from the die 20, an amount ofspring-back Q is produced in the sintered metal blank 13 to increase theinner diameter of the latter (see FIG. 15), so that the core rod 21 canbe extracted from the inner peripheral surface of the sintered metalblank 13 without breaking the hydrodynamic pressure generating grooves 1c (the state of FIG. 14 {circle over (4)}). This completes the bearingbody 1 a.

[0103]FIG. 16 shows the relation between the inner clearance T and outerinterference U and the amount of spring-back Q when said bearing surfaceforming process has been performed on a sintered metal blank of innerdiameterφ3, outer diameter φ6 and width 3 mm. As shown in this figurethere is a certain interrelation between the inner clearance T and outerinterference U and the amount of spring-back Q, it being understood thatwhen the inner clearance T and outer interference U are specified, theamount of spring-back Q is specified. According to experiments, it hasbeen found that at a predetermined groove depth H (2-3 μm), if theamount of spring-back Q is set at 4-5 μm (diametrical value), thesintered metal blank 13 can be extracted from the core rod 21 withoutbreaking the hydrodynamic pressure generating grooves 1 c; thus, it isadvisable to set the inner clearance T and outer interference U in sucha manner as to provide the amount of spring-back Q to that degree. Inaddition, when the radial amount of the spring-back Q of the sinteredmetal blank 13 is greater than the depth H of the hydrodynamic pressuregenerating grooves 1 c, the forming pattern 21 a can be released withoutinterfering with the inner peripheral surface of the sintered metalblank 13. However, even when the radial amount of the spring-back Q ofthe sintered metal blank 13 is less than the depth H of the hydrodynamicpressure generating grooves 1 c and the forming pattern 21 a more orless interferes with the inner peripheral surface of the sintered metalblank 13, it may be enough when the forming pattern 21 a can be releasedfrom the inner peripheral surface of the sintered metal blank 13 withoutbreaking the hydrodynamic pressure generating grooves 1 c, with addingan increase in diameter (radial amount) of the sintered metal blank 13due to the material elasticity of the sintered metal blank 13.

[0104] In addition, after the forming process for the bearing surface 1b has been completed, the bearing surface 1 b may be sized by using anordinary sizing pin (of circular cross section). In this case, the ribs1 d and smooth region n in the bearing surface 1 b are sized by thesizing pin, whereby the percentage of area of surface openings on theirregion becomes lower than that of the region of the hydrodynamicpressure generating grooves 1 c. Also, such a forming process for thebearing surface may be emploied that comprising the steps of formingonly the regin of the hydrodynamic pressure generating grooves by theforming pattern, and then sizing or rotation sizing the other region inthe bearing surface.

[0105] The bearing body 1 a is produced through the processes describedabove and is impregnated with lubricating oil or lubricating grease toretain oil, whereupon the hydrodynamic type porous oil-impregnatedbearing 1 in the form shown in FIG. 1 is completed.

[0106] Comparative tests for shaft deflection were conducted usingcylindrical bearing (a porous oil-impregnated bearings having nohydrodynamic pressure generating grooves formed in the bearing surface)and hydrodynamic type porous oil-impregnated bearings produced by theaforesaid method. The tests were conducted by incorporating testbearings in CD-ROM motors shown in FIG. 18, with a commerciallyavailable CD set therein, the shaft deflection relative to rpm wasmeasured. The results are shown in FIG. 17. It is seen from this figurethat as compared with cylindrical bearing, the hydrodynamic type porousoil-impregnated bearings of the embodiment are effective in suppressingshaft deflection.

[0107] In the above embodiment, the forming process for the bearingsurface has been applied to the sintered metal blank 13; however, it maybe performed in a compacting process for powder metal material. Thiscompacting process is such a process that comprises the steps ofdisposing a forming pin in a die, filling the powder metal materialbetween the forming pin and the die, applying a compacting pressure tothe powder metal material to form into a cylindrical form. In thiscompacting process, it is possible to form a bearing surface as shown inFIG. 1 at the same time of compacting a compacted body, by beingprovided with forming pattern, as shown in FIG. 12B, on the outerperipheral surface of the forming pin. Further, after compaction, thecompacted body can be released from the forming pin while utilizing thespring-back of the compacted body due to removal of the compactingpressure, without any possibility of the bearing surface losing itsshape. The compacted body is sintered, and then it is finished throughsizing, impregnation with oil, etc.

[0108] In addition, it is only necessary that the bearing body beporous; thus, it is not limited to said sintered metal but may, e.g., bea porous body formed by foaming. As blanks therefor, cast iron,synthetic resin, ceramics and the like may be used. Further, in theabove embodiment, the spring-back of the formed body has been utilizedfor releasing the forming pattern; however, the forming pattern may beconstructed such that it can be elastically decreased in diameter. Thus,after the forming of the bearing surface, the forming pattern may beelastically decreased in diameter to be released from the formedproduct. Futher, when forming the bearing surface 11 b shown in FIG. 4,the forming pattern may be shaped as corresponding to the shape of thebearing surface 11 b.

[0109]FIG. 19 shows the state in which a hydrodynamic type porousoil-impregnated bearing 1′ having a plurality of bearing surfaces 1 b′is fixed to a housing 5. The porous oil-impregnated bearing 1′ comprisesa porous body, e.g., a bearing body 1 a′ of sintered metal containingcopper or iron, or both as a main component and oil retained in thepores of the bearing body 1 a′ by impregnation with lubricating oil orlubricating grease.

[0110] The inner peripheral surface of the bearing body 1 a′ is formedwith a plurality of, for example, two, axially spaced bearing surfaces 1b′ opposed to an outer peripheral surface of a shaft to be supported,each of the two bearing sufaces 1 b′ being formed with a plurality ofcircumferentially disposed hydrodynamic pressure generating grooves 1c′. In the same way as shown in FIG. 4, the hydrodynamic pressuregenerating grooves 1 c′ in this embodiment have a V-shaped continuousform having a pair of groove regions, with the grooves in one regioninclined in one direction with respect to the axial direction and thegrooves in the other region inclined in the other direction with respectto the axial direction. The surface openings are distributed on bothregions of the hydrodynamic pressure generating grooves 1 c′ and ribs 1e′ in the bearing surfaces 1 b′. In addition, it is sufficient to formthe hydrodynamic pressure generating grooves 1 c′ in at least one of thebearing surfaces 1 b′.

[0111] The region 1 d′ between the bearing surfaces 1 b′ of the bearingbody 1 a′ has an inner diameter D1 which is greater than the innerdiameter D2 of the bearing surfaces 1 b′ {strictly, the inner diameterof the region of the ribs 1 e′ (corresponding to 1 d in FIG. 9) betweenthe hydrodynamic pressure generating grooves 1 c′}. In this embodiment,the axial section of the region 1 d′ is described with a single arccontinuous to the bearing surfaces 1 b′, the largest diameter portion ofsaid arc being located at the axial center of the region 1 d′. Inaddition, level differences may be provided in the boundaries betweenthe region 1 d′ and the bearing surfaces 1 b′. Further, the axialsection of the region 1 d′ may be described with other curves, besidesan arc, such as ellipse, parabola, etc. It may be described with acombination of two like curves (for example, two arcs), a combination oftwo dissimilar curves (for example, an arc and parabola) or acombination of a curve and a straight line. The largest diameter portionof the region 1 d′ may be deviated to the side associated with onebearing surface 1 b′.

[0112] Further, in this embodiment, the outer diameter D3 of the outerportions 1 f′ corresponding to the two bearing surfaces 1 b′ is smallerthan the outer diameter D4 of the outer portion 1 g′ corresponding tothe region 1 d′ between the bearing surfaces 1 b′ in the bearing body 1a′. When the porous oil-impregnated bearing 1′ is press-fitted in theinner periphery of a housing 5 in the manner shown in the figure,deformation of the bearing surfaces 1 b′ due to the fitting force can beprevented or mitigated, so that substantial accuracy can be obtained.The fixing force can be obtained through the interference between theouter portion 1 g′ and the housing 5. The region 1 d′ is larger indiameter than the bearing surfaces 1 b′ and does not take part insupporting the shaft, so that even if an amount of deformationcorresponding to the fitting force takes place, there is no influence onthe accuracy of the bearing. The difference between the outer diameterD3 of the outer portions 1 f′ and the outer diameter D4 of the outerportion 1 g′ (the difference before press-fitting) is determined suchthat in consideration of the interference with the housing 5 (theinterference of the outer portion 1 g′), the outer portion 1 f′ does notcontact the inner periphery of the housing 5 or provides an amount ofinterference which does not influence the bearing accuracy. In addition,the outer diameter of only one of the two outer portions 1 f′ may bedetermined in the manner described above.

[0113]FIG. 20 shows the flow of oil in an axial section when the shaft 2is supported by the porous oil-impregnated bearing 1′ arranged in themanner described above. As the shaft 2 is rotated, the oil 0 retained inthe bearing body 1 a′ exudes from the axial opposite sides of eachbearing surface 1 b′ to enter the bearing clearance and then it is drawnto the axial center of the bearing clearance by the hydrodynamicpressure generating grooves. The action of drawing the oil 0(hydrodynamic action) increases the pressure of the oil film present inthe bearing clearance, thus forming a lubricating oil film. Thislubricating oil film formed in the bearing clearance supports the shaft2 in a non-contact manner with respect to the bearing surfaces 1 b′without causing unstable vibrations such as whirl. The oil 0 exudinginto the bearing clearance returns to the interior of the bearing body 1a′ through the surface openings in the bearing surfaces 1 b′ under theaction of the generated pressure which accompanies the rotation of theshaft 2, the oil circulating in the interior of the bearing body 1 a′and again exuding into the bearing clearance through the bearingsurfaces 1 b′. In this way, the oil 0 retained in the bearing body 1 a′continuously supports the shaft 2 in a non-contact manner by thehydrodynamic effect while circulating between the bearing clearance andthe bearing body 1 a′.

[0114] Since this porous oil-impregnated bearing 1′ supports the shaft 2in a non-contact manner by the two axially spaced bearing surfaces 1 b′,the shaft 2 can be accurately supported by one bearing. Further, thedrawing action of the hydrodynamic pressure generating grooves 1 c′produces a negative pressure in the space defined between the region 1d′ between the bearing surfaces 1 b′ and the outer peripheral surface ofthe shaft 2 and the oil 0 exudes also from the surface openings on theregion 1 d′ and is fed to the bearing surfaces 1 b′, thereby enhancingthe formation of lubricating oil film in the bearing clearance andincreasing the bearing rigidity. Particularly, in the case where theaxial section of the region 1 d′ is described with an arc (or othercurve) continuous to the bearing surfaces 1 b′as in this embodiment, theoil 0 exuding from the surface openings on the region 1 d′ flows axiallyalong the region 1 d′ until it is effectively fed to the bearingsurfaces 1 b′, a fact which leads to the effective use of oil and theenhancement of formation of lubricating oil film.

[0115] In order to keep such circulation of oil satisfactory, it isdesirable that the surface openings be substantially uniformlydistributed on both regions of the hydrodynamic pressure generatinggrooves 1 c′ and ribs 1 e′ in the bearing surfaces 1 b′. If theproportion of the surface openings (the percentage of area of surfaceopenings) in the surface is decreased, the oil becomes less mobile andreversely if it is increased, the oil becomes more mobile. Further, theviscosity of oil is related to the mobility of oil such that if theviscosity is low, the mobility is high and if it is high, the mobilityis low.

[0116] If the percentage of area of surface openings is high and theviscosity is low, the oil becomes extremely mobile but the oil exudedinto the bearing clearance is readily returned to the interior of thebearing body by the action of the hydrodynamic pressure generatinggrooves, thereby decreasing the hydrodynamic effect. Reversely, if thepercentage of area of surface openings is low and the viscosity is high,the oil becomes extremely immobile, so that, though the pressure of thelubricating oil film increases, the proper circulation of oil is impededand the degradation of oil is accelerated.

[0117] Therefore, there is an optimum range between the percentage ofarea of surface openings and the viscosity of oil which secures theformation of lubricating oil film necessary for supporting the shaft ina non-contact manner and which also secures the appropriate circulationof oil.

[0118] To clarify this optimum ranges evaluation tests were conducted byusing LBP motors. The LBP motors used in the evaluation tests had ashaft diameter of φ4 and a mirror installed therein, the rpm being10,000, the surrounding temperature being 40° C. The results are shownin FIG. 21. In this figure, “◯” indicates the absence of problems in1,000-hour continuous running endurance test. And “Δ” indicates thattroubles occurred, during 500-1,000 hours, such as an increase in shaftdeflection (5 μm or above), an increase in torque=a decrease in rpm (therpm failed to increase to 10,000 rpm) and abnormal sound and that normaloperation was impossible. The mark “X” indicates that such troublesoccurred within 500-1,000 hours.

[0119] It is seen from the above evaluation tests that the optimum rangeof the percentage of area of surface openings and the oil viscosity (theregion where there is no “X”) is the area surrounded by solid line inFIG. 21, which area satisfies the following conditions:

[0120] a) The percentage of area of sufade openings on the bearingsurface including the region of the hydrodynamic pressure generatinggrooves is not less than 2% but not more than 20%;

[0121] b) The kinematic viscosity of retained oil at 40° C. is not lessthan 2 cSt;

[0122] c) The percentage of area of surface openings on the bearingsurface and the kinematic viscosity of oil at 40° C. satisfy therelation

(3/5)A−1≦η≦(40/6)A+(20/3)

[0123] where A; percentage of area of surface openings [%]

[0124] η; kinematic viscosity of oil at 40° C. [cSt]

[0125] Selecting the percentage of area of surface openings and the oilviscosity within such range ensures formation of a sufficientlubricating oil film to support the shaft in a non-contact manner andits proper circulation, so that high rotation accuracy and long life canbe attained.

[0126] There is an optimum range of ratio of the depth (h) of thehydrodynamic pressure generating grooves to the size of the bearingclearance (radial clearance: c) and it is believed that with valuesoutside the range, the sufficient hydrodynamic effect cannot beobtained. To clarify this optimum range, evaluation tests were conductedby replacing the shaft of the LBP motor by a longer one to allowmeasurement of shaft deflection. The rpm was 10,000 and the test ambientatmosphere was at ordinary temperatures and humidity, and the LBP motorwasφ4, and did not have a mirror installed therein. In addition, theshaft defection was measured with a non-contact type displacement gauge.

[0127] Under the above conditions, values of the shaft deflectionrelative to the c/h (c; radial clearance, h; groove depth) were plotted,and the results shown in FIG. 22 were obtained. It is seen from FIG. 22that when the c/h is in the range of 0.5-4.0, then the shaft deflectionis not more than 5 μm, but if it is less than 0.5 or greater than 4.0,then the shaft deflection is not less than 5 μm. Therefore, to maintainhigh accuracy, it is desirable that the c/h be in the range of 0.5-4.0.In addition, it is desirable that the size c of the bearing clearance(radial clearance) be such that when the radius of the shaft is R, thenthe c/R is in the range of 1/2,000-1/400.

[0128] A hydrodynamic type porous oil-impregnated bearing 1″ shown inFIG. 23 also has a plurality of bearing surfaces; however, the shape ofthe bearing surfaces differs from that of the hydrodynamic type porousoil-impregnated bearing 1′ shown in FIG. 19.

[0129] Each of the bearing surfaces 1 b″ of the porous oil-impregnatedbearing 1″ in this embodiment comprises a first region m1 in which aplurality of hydrodynamic pressure generating grooves 1 c 1 inclined inone direction with respect to the axial direction are circumferentiallydisposed, a second region m2 which is axially spaced from said firstregion m1 and in which a plurality of hydrodynamic pressure generatinggrooves 1 c 2 inclined in the other direction with respect to the axialdirection are circumferentially disposed, and an annular smooth region ndisposed between the first and second regions m1 and m2. The ribs 1 e 1of the first region m1 and the ribs 1 e 2 of the second region m2continuous to the smooth region n. When a relative rotation is producedbetween the bearing body 1 a″ and the shaft, the hydrodynamic pressuregenerating grooves 1 c 1 and 1 c 2 formed in the first and secondregions m1 and m2 in a mutually reversely inclined manner draw oil intothe smooth region n to collect the oil in the smooth region n, wherebythe oil film pressure in the smooth region n is increased. Furthermore,since the smooth region n has no grooves formed therein, the effect offormation of lubricating oil film in this region is high, and inaddition to the ribs 1 e 1 and 1 e 2, the smooth region n provides asupport surface for supporting the shaft, whereby the support area isincreased and so is the bearing rigidity. Further, the axial section ofthe region 1 d″ between the bearing surfaces 1 b″ is described with anaxial straight line, and the boundaries between the region 1 d″ and thebearing surfaces 1 b″ form level differences 1 h. In addition, the axialsection of the region 1 d″ may be described with a combination of twostraight lines inclined with respect to the axial direction (V-shapedtype).

[0130] In addition, as in the case of the hydrodynamic type porousoil-impregnated bearing 1′ shown in FIG. 19, the inner diameter of theregion 1 d″ is greater than that of the bearing surfaces 1 b′, and theouter diameter of the outer portions 1 f′ corresponding to the bearingsurfaces 1 b″ is smaller than that of the outer portion 1 g″corresponding to the region 1 d″.

[0131] Comparative tests on press-fitting in a housing and rotationaccuracy comparative tests were conducted. The results are describedbelow.

[0132] (1) Comparative tests on Press-fitting in Housing

[0133] Comparative article: Constructed such that it has a singlebearing surface having hydrodynamic pressure generating grooves formedtherein. Two test bearings were produced, whose inner diameter beforepress-fitting was φ3.006, and they were press-fitted in a housing withan interference of 18 μm, the correcting pin diameter being φ3.000 mm.

[0134] Embodied article: Constructed such that it has two bearingsurface each having hydrodynamic pressure generating grooves formedtherein. The test bearing was press-fitted in a housing under the sameconditions as above.

[0135] Test results: In the case of the comparative article, the twobearings had part of their hydrodynamic pressure generating groovescollapsed. The tests were conducted with the bearings installed inmotors, and the rotation was unstable, producing a shaft deflection andthe like which are worse than in the case of ordinary cyrindricalbearings (bearings which have no hydrodynamic pressure generatinggrooves formed in their bearing surfaces). The cause of collapse of partof the hydrodynamic pressure generating grooves seems to be the localthickening of material in the test bearings (same with bearingproducts); therefore, it is believed that the correcting force from thecorrecting pin acted heavily on part of the hydrodynamic pressuregenerating grooves. In contrast thereto, in the embodied article,although the groove depth was found decreased as a whole (from 4 μm to3.5 μm), there was observed no phenomenon in which part thereof wascollapsed. When the bearing was installed in a motor and the shaftdeflection was measured, it exhibited an excellent performance; theshaft deflection was not more than 2 μm at 2,000-15,000 rpm.

[0136] (2) Rotation Accuracy Comparative Tests

[0137] Comparative article: Constructed such that it has two bearingsurfaces each having no hydrodynamic pressure generating grooves formedtherein.

[0138] Embodied article: Constructed such that it has two bearingsurfaces each having hydrodynamic pressure generating grooves formedtherein (the construction being shown in FIG. 19).

[0139] Test results: The test results are shown in FIG. 24. As shown inthis figure, the embodied article, as compared with the comparativearticle, exhibited a superior performance {the mark (▪) indicatesmeasured data for the embodied article and () for the comparativearticle).

[0140] In addition, the hydrodynamic type porous oil-impregnated bearinghaving a plurality of bearing surfaces can be produced by the aforesaidmethod using a core rod or forming pin in which forming patternscorresponding to the shape of the bearing surfaces are formed in aplurality of places on the outer peripheral surface thereof.

What is claimed is:
 1. A hydrodynamic type porous oil-impregnatedbearing comprising a porous bearing body formed with bearing surface onan inner peripheral surface thereof, and oil retained in pores of saidbearing body by impregnation of lubricating oil or lubricating grease,wherein said bearing surface has a first region in which a plurality ofhydrodynamic pressure generating grooves inclined in one direction withrespect to the axial direction are circumferentially disposed, a secondregion which is axially spaced from the first region and in which aplurality of hydrodynamic pressure generating grooves inclined in theother direction with respect to the axial direction arecircumferentially disposed, and an annular smooth region positionedbetween the first and second regions.
 2. A hydrodynamic type porousoil-impregnated bearing as set forth in claim 1, wherein said bearingbody is formed of a sintered metal.
 3. A hydrodynamic type porousoil-impregnated bearing as set forth in claim 2, wherein said sinteredmetal contains copper or iron, or both as a main component.
 4. Ahydrodynamic type porous oil-impregnated bearing as set forth in claim1, wherein percentage of area of surface openings on said smooth regionis lower than that of said first and second regions.
 5. A hydrodynamictype porous oil-impregnated bearing as set forth in claim 1, wherein theaxial opposite sides of said bearing surface are tapered surfaces havinginner diameter increased toward the bearing ends.
 6. A hydrodynamic typeporous oil-impregnated bearing as set forth in claim 1, wherein theinner peripheral surface of said bearing body is formed with a pluralityof said bearing surfaces axially spaced from each other.
 7. Ahydrodynamic type porous oil-impregnated bearing as set forth in claim1, wherein said hydrodynamic pressure generating grooves of said firstregion and said hydrodynamic pressure generating grooves of said secondregion are symmetric with respect to the axial center of said bearingsurface.
 8. A hydrodynamic type porous oil-impregnated bearing as setforth in claim 1, wherein a region of said hydrodynamic pressuregenerating grooves and the other region in said bearing surface aresimultaneously formed by a forming pattern having a shape correspondingto said bearing surface.
 9. A hydrodynamic type porous oil-impregnatedbearing comprising a porous bearing body being formed with a pluralityof axially spaced bearing surfaces on an inner peripheral surfacethereof, at least one of said plurality of bearing surfaces havinginclined hydrodynamic pressure generating grooves, inner diameter of aregion between said bearing surfaces being greater than inner diameterof said bearing surfaces, and oil retained in pores of said bearing bodyby impregnating of lubricating oil or lubricating grease.
 10. Ahydrodynamic type porous oil-impregnated bearing as set forth in claim9, wherein said bearing body is formed of a sintered metal.
 11. Ahydrodynamic type porous oil-impregnated bearing as set forth in claim10, wherein said sintered metal contains copper or iron, or both as amain component.
 12. A hydrodynamic type porous oil-impregnated bearingas set forth in claim 9, wherein the boundaries between said bearingsurfaces and the region between said bearing surfaces are of leveldifferences.
 13. A hydrodynamic type porous oil-impregnated bearing asset forth in claim 9, wherein the axial section of the region betweensaid bearing surfaces is drawn with a curve which is continuous to saidbearing surfaces.
 14. A hydrodynamic type porous oil-impregnated bearingas set forth in claim 13, wherein said curve is an arc such that itsdiameter is greatest at the axial center of the region between saidbearing surfaces.
 15. A hydrodynamic type porous oil-impregnated bearingas set forth in claim 9, wherein outer diameter of an outer portion ofsaid bearing body corresponding to at least one of said bearing surfacesis smaller than outer diameter of an outer portion of said bearing bodycorresponding to the region between said bearing surfaces.
 16. Ahydrodynamic type porous oil-impregnated bearing as set forth in claim9, wherein a region of said hydrodynamic pressure generating grooves andthe other region in said bearing surface are simultaneously formed by aforming pattern having a shape corresponding to said bearing surface.17. A method of producing a hydrodynamic type porous oil-impregnatedbearing comprising a porous bearing body being formed with bearingsurface on an inner peripheral surface thereof, said bearing surfacehaving inclined hydrodynamic pressure generating grooves, and oilretained in pores of said bearing body by impregnation of lubricatingoil or lubricating grease, said method comprising the steps of:inserting a forming pattern in an inner peripheral surface of acylindrical porous blank, said forming pattern having a first formingportion for forming a region of said hydrodynamic pressure generatinggrooves and a second forming portion for forming the other region insaid bearing surface, applying a compacting pressure to said porousblank to press the inner peripheral surface of said porous blank againstsaid forming pattern, thereby simultaneously forming the region of saidhydrodynamic pressure generating grooves and the other region in saidbearing surface on the inner peripheral surface of said porous blank.18. A method of producing a hydrodynamic type porous oil-impregnatedbearing as set forth in claim 17, wherein said bearing surface has afirst region in which a plurality of hydrodynamic pressure generatinggrooves inclined in one direction with respect to the axial directionare circumferentially disposed, a second region which is axially spacedfrom the first region and in which a plurality of hydrodynamic pressuregenerating grooves inclined in the other direction with respect to theaxial direction are circumferentially disposed, and an annular smoothregion positioned between the first and second regions.
 19. A method ofproducing a hydrodynamic type porous oil-impregnated bearing as setforth in claim 17, wherein said bearing surface has a first region inwhich a plurality of hydrodynamic pressure generating grooves inclinedin one direction with respect to the axial direction arecircumferentially disposed, a second region which is axially continuousto the first region and in which a plurality of hydrodynamic pressuregenerating grooves inclined in the other direction with respect to theaxial direction are circumferentially disposed.
 20. A method ofproducing a hydrodynamic type porous oil-impregnated bearing as setforth in claim 17, wherein said porous blank is formed of a sinteredmetal.
 21. A method of producing a hydrodynamic type porousoil-impregnated bearing as set forth in claim 20, wherein said sinteredmetal contains copper or iron, or both as a main component.
 22. A methodof producing a hydrodynamic type porous oil-impregnated bearing as setforth in claim 17, wherein after forming said bearing surface, releasingsaid forming pattern from the inner peripheral surface of said porousblank while utilizing the spring-back of said porous blank due toremoval of said compacting pressure.
 23. A method of producing ahydrodynamic type porous oil-impregnated bearing comprising a porousbearing body being formed with bearing surface on an inner peripheralsurface thereof, said bearing surface having inclined hydrodynamicpressure generating grooves, and oil retained in pores of said bearingbody by impregnation of lubricating oil or lubricating grease, saidmethod comprising the steps of: disposing a forming pattern in a die,said forming pattern having a first forming poriton for forming a regionof said hydrodynamic pressure generating grooves and a second formingportion for forming the other region in said bearing surface, fillingpowder metal material between said forming pattern and said die,applying a compacting pressure to said powder metal material to form acylindrical compacted body, while simultaneously forming a region ofsaid hydrodynamic pressure generating grooves and the other region insaid bearing surface on an inner peripheral surface of said compactedbody by said forming pattern.
 24. A method of producing a hydrodynamictype porous oil-impregnated bearing as set forth in claim 23, whereinsaid bearing surface has a first region in which a plurality ofhydrodynamic pressure generating grooves inclined in one direction withrespect to the axial direction are circumferentially disposed, a secondregion which is axially spaced from the first region and in which aplurality of hydrodynamic pressure generating grooves inclined in theother direction with respect to the axial direction arecircumferentially disposed, and an annular smooth region positionedbetween the first and second regions.
 25. A method of producing ahydrodynamic type porous oil-impregnated bearing as set forth in claim23, wherein said bearing surface has a first region in which a pluralityof hydrodynamic pressure generating grooves inclined in one directionwith respect to the axial direction are circumferentially disposed, asecond region which is axially continuous to the first region and inwhich a plurality of hydrodynamic pressure generating grooves inclinedin the other direction with respect to the axial direction arecircumferentially disposed.
 26. A method of producing a hydrodynamictype porous oil-impregnated bearing as set forth in claim 23, whereinsaid powder metal material contains copper or iron, or both as a maincomponent.
 27. A method of producing a hydrodynamic type porousoil-impregnated bearing as set forth in claim 23, wherein after formingsaid compacted body and said bearing surface thereof, releasing saidforming pattern from the inner peripheral surface of said compacted bodywhile utilizing the spring-back of said compacted body due to removal ofsaid compacting pressure.